Past attempts at the modeling of water exchanges in the heart and other well-perfused organs have utilized relatively simple, usually single compartmental models. Neither compartmental models nor simple axial distributed models can be fitted well to high resolution data. Both model forms give estimates of the capillary permeability-surface area products which are unrealistic, but for different reasons. For solutes that are very nearly flow-limited in their rates of blood-tissue exchange, the PS estimates differ by the two approaches. Osmotic transient water fluxes tend to be limited by the transmembrane fluxes of hydrophilic solutes, not by water permeability, therefore models accounting for any net water flux must also account for the solute fluxes. In this project we will bring together the descriptions of the phenomena governing water and solute fluxes in well-perfused organs, and thereby reconcile the disparate estimates of permeabilities obtained by using different experimental an d analytical techniques. This project extends previous projects on tracer water exchanges and tracer and non-tracer solutes of a wide range of molecular sizes; the later models will describe tracer and non-tracer water during net fluxes of non-tracer water.